Chat leaders Akira Sawa of Johns Hopkins University, Nick Brandon of Wyeth Research, and Ty Cannon of UCLA led us in a wide-ranging discussion of all things DISC1 on January 13, 2009, including the following:

The molecular biology of DISC1 and its various isoforms.

Approaches to studying disruptions of the gene in rodents and other organisms.

The value of DISC1 in efforts to categorize and treat neuropsychiatric disease.

As we await the posting of the transcript, we invite you to read the background text and to offer your comments.

It has been a very productive two years in the DISC1 area since the previous Schizophrenia Research Forum roundtable (see Porteous’s background text and Sawa’s post-meeting summary), and as the field is poised for its next batch of publications, especially in generation of model animals for DISC1 (Wang et al., 2008) and identification of molecular pathways involving DISC1 and other genetic risk factors (e.g., Kamiya et al., 2008; see SRF news story). Therefore, we believe that it is again a good time to get a group together to look at the progress that has been made and to make suggestions on areas in which we need to work harder.

Genetically, DISC1 is a major risk factor for a wide range of psychiatric disorders, including schizophrenia (Chubb et al., 2008). A rare variant with strong biological impact associated with the disorders in the DISC1 locus was identified from a large Scottish Pedigree (St. Clair et al., 1990). Some, but not all, association studies have supported that DISC1 is a risk factor for schizophrenia; nonetheless, such association becomes more promising when specific disease-related endophenotypes are considered (Cannon et al., 2005). It is still an excellent question as to what are the nature and effects of DISC1 variants in psychiatric genetics. This question is crucially associated with an issue of how we can utilize DISC1 genetically engineered organisms/animals in a translational sense.

From a biology viewpoint, DISC1 is a multifunctional protein localized to several distinct subcellular compartments (Ishizuka et al., 2006). DISC1 interacts with many proteins of importance (Camargo et al., 2007) and seems to function as an anchoring protein to regulate distinct cascades either at certain developmental time-points or in response to various stimuli. Thus, important questions in DISC1 biology are: what is the nature of disease-relevant DISC1 cascades or molecular pathways, and how are these cascades distinctly regulated in a context-dependent manner (e.g. temporally and spatially)? The complexity of DISC1 isoforms is still unsolved and could be critical for this last question. For example, when we consider the recently appreciated centrosomal and synaptic roles of DISC1, where does the underlying versatility derive from? Is it due to different DISC1 isoforms or due to the same species playing different roles at different developmental stages (or both)?

Based on this platform, the following points should be considered for this discussion.

1) The complexity of the DISC1 molecule (isoforms, potential role for antisense transcripts and fusion transcripts)

3) The role for model organisms (mice, flies, zebrafish, etc.) in DISC1 research, especially their translational utilities. With the burgeoning number of DISC1 mice, can we rationalize a path forward? Are all these models in all cases simply interfering with a key neurodevelopmental protein or are they really telling us something about the human disease? In terms of non-mouse models, what are we learning and does it have any relevance to humans?

If indeed genetic variation in the DISC1 gene confers risk...
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If indeed genetic variation in the DISC1 gene confers risk for mental illness, the effects of risk-associated polymorphisms should be detectable at the level of pre-mRNA processing, transcription, post-translational modification of proteins or their subcellular distribution, as these are the most proximal intermediate phenotypes to risk-associated alleles. So far, however, the results of studies searching for molecular correlates of genetic associations have been inconclusive and the mechanisms remain unknown. Although reduced expression of DISC1 mRNA was found in the lymphoblastoid cell lines of family members with the translocation (Millar et al., 2005) and in bipolar disorder patients with a putative risk haplotype (Maeda et al., 2006), no changes were detected in the brain tissue of unrelated patients with schizophrenia or in individuals carrying risk genotypes (Lipska et al., 2006; Dean et al., 2007). Furthermore, in an allele-specific expression assay, two coding SNPs in DISC1 did not show an effect on DISC1 mRNA in the brain tissue (Hayesmoore et al., 2008).

On the other hand, Sawamura et al. (2005) reported enrichment of certain shorter, though uncharacterized, isoform(s) of DISC1 in the nuclear fraction of brain tissue extracts in patients with schizophrenia and major depression. This finding raised an interesting possibility that DISC1 mRNA and/or protein processing and distribution might be altered in patients with major mental illness. Indeed, further evidence for this premise comes from our own data showing that immunoreactivity of the same short, and as yet unidentified, DISC1 isoform(s) is higher in the hippocampus of patients with schizophrenia and predicted by risk-associated SNPs (Lipska et al., 2006). Furthermore, studies probing cells, mouse and human brain protein extracts with a number of well-characterized antibodies have provided ample evidence for the existence of a large variety of unknown DISC1 protein forms, suggesting that DISC1 processing mechanisms are complex and might be altered in mental illness and affected by risk related genetic variation (summarized in Ishizuka et al., 2007). Studies using well characterized, high quality postmortem brain tissues are critical for elucidating how risk-associated alleles in the DISC1 gene affect mRNA processing. Given that schizophrenia is a neurodevelopmental disorder, it might be essential to extend these studies to an early developmental period. Such findings may potentially be crucial for understanding apparent inconsistencies in results of association studies for DISC1, which may involve true genetic heterogeneity (as different SNPs in the same gene may result in the same molecular phenotype).

What is the current situation in linking DISC1 to psychiatric disease?
1. There is the Scottish pedigree where DISC1 disruption could be genetically linked to clinical phenotypes of chronic mental diseases

2. Genetic association studies have associated polymorphisms within DISC1 to different psychiatric diseases but these results are sometimes weak and seem inconsistent

What are the problems?
1. Does DISC1 have anything to do with sporadic cases of chronic mental disorder (CMD)?

2. If yes, what is the molecular mechanism of action?

3. Given the multitude of molecular interactions of DISC1, is there a smallest common molecular denominator of DISC1 dysfunction with regard to behavioral (or other) phenotypes?

General comments:
I agree that it is of paramount importance to have a valid DISC1-related animal model showing any kind of phenotype, preferably one with a clear and unequivocal behavioral phenotype, and, even better, a clear and unequivocal neuropathological phenotype.

However, I consider it equally important to link DISC1 to sporadic cases of chronic mental diseases because otherwise the animal models would not directly address an important clinical issue. It is unlikely that a role of DISC1 can be established in the majority of sporadic CMD cases given the heterogeneity of human psychiatric diseases. A role of DISC1 in a subset of sporadic cases is exciting enough! How could DISC1 play a role in causing sporadic CMD? Any posttranscriptional and posttranslational modifications in DISC1 could account for this. Starting from the role of differential DISC1 splice variants, to posttranslational DISC1 modifications like phosphorylation, degradation or others.

We have shown that recombinantly expressed full length DISC1 in eukaryotic cells assembles into multimeric complexes. In a cell-free in vitro system, a relevant recombinant DISC1 fragment expressed in E. coli interacted with NDEL1 only as an octameric complex in solution, but not as dimers or high molecular weight multimers (Leliveld et al., 2008). In the same paper, we demonstrated that DISC1 is an aggregation-prone protein that in its insoluble state would loose binding to NDEL1. In fact, we were able to identify a subset of patients with variable clinical phenotypes in the Stanley Foundation Consortium Collection with insoluble DISC1. In my view, this indicates that misassembly of DISC1 could account for its dysfunction in sporadic cases with CMD.

I do not think that this is the end of the story but just demonstrating one mechanism of how posttranslational modifications of DISC1 can affect its function and thereby lead to disease. Remember Alzheimer's disease, where >95 percent of cases are sporadic, i.e., no mutations can be identified, but still amyloid-β and tau are post-translationally modified and considered causative for the clinical phenotype.

DISC1 appears to be related to a number of psychiatric phenotypes, bridging across traditional diagnostic boundaries and influencing key clinical manifestations of illness including neurocognitive function, symptom domains, and neuroimaging parameters. In addition, we (Burdick et al., 2008) and others have recently noted that the DISC1 interactome is also critically related to this broad array of phenotypes, and the specific genetic relationship may be predicated on different DISC1 genetic backgrounds. Therefore, the examination of single genes, and proteins, may not be sufficient to fully assess the biological effects of this system on behavioral phenotypes.

This complexity further complicates the search for relevant animal models, as most genetic models are based upon single gene perturbations. Animal models that incorporate perturbations across multiple genes and that can take into account developmental timing and produce relevant behavioral phenotypes would be a great step forward. In particular, the relationship of DISC1 and its binding partners to treatment response has not been investigated at either the preclinical or clinical level. For a protein with such diverse effects on psychosis not to have an effect on treatment response would be unexpected, but this remains an empirical question to be addressed. The challenges of conducting pharmacogenetics research mirror some of the challenges surrounding the development of animal models. The need for consistent treatment approaches, for careful and reliable assessment of treatment outcomes, yet with increasingly large sample sizes, are currently limiting factors for the field. However, at the end of the day, the key issue surrounding any of this work will be whether or not we can significantly influence the treatment outcome of our patients, and much work remains to be completed before appropriate consideration of this element of the DISC1 research portfolio.

I am unfortunately unable to attend the live discussion, but I am pleased to note this topic being revisited. All of the evidence gathered since the last SRF discussion supports my view that the study of DISC1 genetics and biology has much to offer the field. I have followed each of the topic questions with a short comment. I hope you find these helpful and that the discussion is productive. Also, if you haven’t already done so, check out the SRF meetings calendar or go straight to the Keystone Meetings website and register for the following meeting. There is a great lineup of speakers, loads of discussion, plus fantastic snow sports.

1) The complexity of the DISC1 molecule (isoforms, potential role for antisense transcripts and fusion transcripts). This is clearly an important issue. DISC1 is a large multi-exon gene with evidence for multiple splice forms. I note Barbara Lipska’s comments regarding the lack of evidence for reduced DISC1 expression in schizophrenia brains, but believe that more detailed characterization during development and in relation to pathology is still warranted. The effect of normal genetic variation and of putative pathogenic variants is likewise relevant. We lack biopsy material from any of the t(1;11) subjects through which DISC1 was identified to test for reduced levels in the brain, but if we extrapolate from cell culture studies and half normal levels of DISC1 are highly penetrant for major mental illness, then our assays need to be very sensitive to detect modest, but still potentially relevant reductions in DISC1 levels in the generality of schizophrenia and other related brain disorders.

The complexity of neuronal types alone and the recognition of the multi-functional nature of the DISC1 complex make these self-evidently key considerations. In this regard, cell and animal model systems must be combined as the scope for human studies is obviously limited. Past limitations may, however, be partially circumvented in the future by application of iPS technology and related pluripotent stem cell approaches. In the meantime, animal models have much to offer that is otherwise obscured from view by the limited access to appropriate and relevant human tissue.

3) The role for model organisms (mice, flies, zebrafish, etc.) in DISC1 research, especially their translational utilities. With the burgeoning number of DISC1 mice, can we rationalize a path forward? Are all these models in all cases simply interfering with a key neurodevelopmental protein or are they really telling us something about the human disease? In terms of non-mouse models, what are we learning and does it have any relevance to humans?

A model is just that. The key issue is to define the hypothesis and the experimental approach to test it.
We are at an early stage with all these models. Some consistent themes are emerging from the mouse studies—brain developmental abnormalities and working memory defects—but different studies report on different phenotypes so cross-comparisons are only partial. Each of the several mouse models already available have their value. I believe we still lack a true null. This would be valuable as a test bed on which to test the known and emerging human mutations.
I can see some useful insights coming also from Danio and Drosophila which each offer specific experimental advantages.

4) Nature and effects of DISC1 variants on phenotypes/endophenotypes beyond DSM diagnostic criteria. Only the most stubborn skeptic would deny that mutation of DISC1 can be causally related to major mental illness, but as the current state of genetic linkage, candidate gene association, GWAS, CNV, and molecular cytogenetics studies tell us, the full picture of genetic architecture underlying schizophrenia and related brain disorders is barely started. Nor can we estimate the net contribution of DISC1 and DISC1 complex genetic variants to risk, at the individual or population level, but that is true for all current candidates. I would argue, however, that even at this early stage, it is highly likely that a wide range of mutation classes (regulatory, deletion, insertion, missense, nonsense) have already or will soon be found for DISC1. Large-scale resequencing on well-characterized samples (normal and disease) is warranted and likely to be highly informative with regard to structure-function insights.

5) Should we expect any therapeutic breakthroughs for schizophrenia and other neuropsychiatric disorders via DISC1 research?

The gap between expectation and delivery is great, but as a platform from which to develop a program of rational drug development, the DISC1 complex presents as good an opportunity as any currently available. The big question is how and when academia and industry will partner each other successfully in the pursuit. We are not helped in this regard by the current global economic climate and the associated risk-averse nature of research investment. But we have to try.

I think that it has been a very productive couple of years...
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I think that it has been a very productive couple of years since your last forum, and DISC1 has established itself as one of the most interesting genes in psychiatric genetics.

The recent work examining CNVs in schizophrenia over the last 12 months indicates 1) that there are likely to be very many other rare high penetrant mutations which, like DISC1 mutations, predispose individuals and families to high risk of schizophrenia and other neuropsychiatric disorders, and 2) that as with DISC1 there is unexpected clinical phenotypic diversity. Most of these CNVs, whether deletions or duplications, disrupt some or many aspects of CNS function at all stages of brain development. In some cases these mutations are so rare that it will be difficult to show statistical association with schizophrenia and evidence of involvement will need to be circumstantial. The great advantage of theDISC1 mutation is that so many members of the family with the mutation are affected that the association is highly unlikely to be due to chance.

Detailed study of all aspects of DISC1 is likely to continue to not only be rewarding in its own right but to help set the framework and ground rules as to how best to study the many other rare mutations that predispose to schizophrenia and related disorders that are now being identified.

Also very interesting is the accumulating genetic evidence implicating a number of direct DISC1 interactors in schizophrenia; the most convincing and interesting are PDE4B, Nde1, and PCM1.

I fully agree with aims 1 and 2 of Sawa and Brandon's discussion text, although they are not my area of expertise. Modeling poses special challenges since the range of phenotypes associated with neuron expressed gene disruptions is so extensive. Key issues will be to try to determine the genetic and environmental factors that influence penetrance and expressivity. These could be studied in rodent and non-rodent models e.g., zebra fish. One of the most interesting findings to emerge in DISC1 mice is a GABAergic deficit. This could be a key translational target, since GABAergic deficits are consistently found in schizophrenia; it is possible that a whole range of upstream genetic lesions can cause downstream GABAergic deficits, and these are responsible for some of the cognitive and perceptual problems that are widespread in schizophrenia. In my opinion, detailed study of these downstream anomalies in DISC1 is as important as the upstream biochemistry.

It is impossible to predict whether therapeutic breakthroughs can be expected. Three potential interesting leads give hope: 1) cAMP appears to affect PDE4B, NUDEL, and DISC1 interaction, so drugs like Rolipram could be of importance for the DISC1 cascade as a whole; 2) GABAergic drugs should be examined; 3) it is possible that prenatal nutritional status could affect penetrance and expressivity of many mutations causing schizophrenia. This can be examined in DISC1 models and if demonstrated could lead to potential preventative interventions, e.g., micronutrient supplementation.

After much anticipation, a variety of mutant and transgenic DISC1 mouse lines from several research groups has been published over the last two years. Most of these attempt to mimic the original t(1:11) DISC1 mutation by transgenic expression of truncated DISC1 proteins. The Gogos group introduced a targeted premature stop codon into exon 8 of the endogenous DISC1 of the 129 mouse strain (Koike et al., 2006; Kvajo et al., 2008), which itself was found to carry a spontaneous premature stop codon in DISC1 exon 7 (Koike et al., 2006). The Pletnikov, Sawa and Silva groups generated transgenic lines that have constitutive (Hikida et al., 2007) or inducible (Li et al., 2007; Pletnikov et al., 2008) CaMKIIa promoter-driven expression of truncated human DISC1. The St Clair group generated a BAC transgenic line expressing truncated mouse DISC1 under the influence of the (still not formally identified) DISC1 promoter (Shen et al., 2008). Taking a different tack, the Roder group screened an archive of ENU-mutagenized mice for mutations in DISC1, and found two missense mutations within PDE4B binding sites (Clapcote et al., 2008).

In most cases, the mouse lines have had a predominantly C57BL/6 genetic background (Koike et al., 2006; Kvajo et al., 2008; Hikida et al., 2007; Li et al., 2007; Clapcote et al., 2008), which is an important consideration when comparing the phenotypes of each line. Although the mutations may have different mechanisms, their effects on brain morphology and behavior appear similar; with fairly consistent effects on working memory, brain morphology and prepulse inhibition (see Table 1, Shen et al., 2008). There are still no DISC1 knockout mice available, but we await these with bated breath. It would be interesting to cross DISC1 truncation transgenics with the DISC1 knockouts when they come along, to test whether having a part of DISC1 is better than none at all.

I have tried to answer some of the questions asked by the...
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I have tried to answer some of the questions asked by the discussants.

With the burgeoning number of DISC1 mice can we rationalize a path forward?

Combining available animal DISC1 models with other genetic mouse models and/or environmental factors appears as an obvious next thing to try to answer some fundamental questions about psychiatric disorders. Again, we will be using DISC1 as a great discovery tool.

Are all these models in all cases simply interfering with a key neurodevelopmental protein or are they really telling us something about the human disease?

I would suggest both. In other words, by interfering with a key developmental protein, we are beginning to understand better the biology and functions of DISC1 and potential pathogenic process of psychiatric disorders when the functions are perturbed.

In terms of non-mouse models what are we learning and does it have any relevance to man?

I often feel that many investigators are either overly optimistic or too negative towards animal models. Models are just what they are—models. Perhaps we should stop worrying about how well our models reflect/mimic a disease or a symptom. If they are helpful to answer specific BIOLOGICAL questions related to genes, environmental factors or the pathways, we can and should use these models. Fruit fly-based models have helped to uncover several key fundamental biological functions and principles. Are they are relevant to human diseases? Yes, they are as long as the disease has the molecular underpinning. Since we seem to agree that psychiatric diseases have biological roots, we will need to use animal models to experiment. Will animal models be sufficient for describing the psychiatric disease in its entirety? No, they won’t, but what they may show to us is, for example, that there may be several common biological processes that underlie different psychiatric disorders, supporting what is emerging from psychiatric genetics that many genes are equally associated with different clinical diagnoses.

The study of Disc1's biological role gives us hope to comprehend much deeper neurobiological processes of such complex psychopathologies as schizophrenia or bipolar disorder. Analysis of Disc1 interactions with many other proteins (Camargo et al., 2007) will help us to dissect the function(s) of each molecular player. Excellent recent reviews by Chubb et al., 2008, and Wang et al., 2008, are very helpful in fully comprehending the biology of Disc1. I will give you some of my comments regarding the preliminary questions for the DISC1 roundtable discussion:

1. The complexity of the DISC1 molecule (isoforms, potential role for antisense transcripts and fusion transcripts).
What is the role of multiple isoforms of Disc1? I think the comparison of Disc1 structure, functions between biological species from an evolutionary point of view, might be helpful at least to understand the origins of multiple Disc1 isoforms.

A very exciting area of investigation is to find the biological meaning of Disc1 environment interactions, whereby psychopathological cascades could be triggered by different manipulations of prenatal (like immune activator Poly I:C) or postnatal stress (e.g., chronic social stress). Exposure of different Disc1 mouse mutants with genetic predisposition to the development of schizophrenia or bipolar disorder to the different types of stressful environment is a very productive approach.

3. The role for model organisms (mice, flies, zebrafish, etc.) in DISC1 research, especially in their translational utilities. With the burgeoning number of DISC1 mice, can we rationalize a path forward? Are all these models in all cases simply interfering with a key neurodevelopmental protein or are they really telling us something about the human disease? In terms of non-mouse models, what are we learning and does it have any relevance to humans?

I think that analysis of many animal models will provide better understanding of Disc1’s biological role. Having multiple Disc1 mutants with different alterations of Disc1 protein will help us to identify multiple functions of Disc1 and define its role(s) within the whole psychopathological process underlying schizophrenia or bipolar disorder. Moreover, analysis of Disc1 interactor(s) mouse mutant models will discover new functions of known proteins or possibly reveal new proteins involved in the same psychopathological pathways. It will be interesting to find how conserved the role of Disc1 is among different biological species, and non-mouse models could be very helpful.

Even though we mostly focus on psychopathologies, expanded behavioral procedures could reveal new phenotypes/endophenotypes of Disc1 variants beyond DSM diagnostic criteria. For instance, Disc1 could play an important role in navigation processes, and decoding their role in place cells would be an exciting finding.

5. Should we expect any therapeutic breakthroughs for schizophrenia and other neuropsychiatric disorders via DISC1 research?

Yes, our combined efforts of Disc1 investigations will bring us a lot of new discoveries in the near future.

I think that there is little doubt that DISC1 is a fantastic tool to probe the mechanism(s) underlying schizophrenia and mental disease in general, and I am convinced that many exciting breakthroughs lie ahead. However, I would like to share some cautioning thought, following up on comments from my colleague Carsten Korth: What if DISC1 has absolutely nothing to do with sporadic cases of mental disease (SMD), because it is upstream of the damage/alterations that occur in these diseases? For example, let´s suppose that DISC1, through its interaction with PDE4B, plays an essential regulatory role in the flow of cAMP to some specific cell location in a given set of neurons, in turn, that cAMP flow regulates the activity of a given effector, "X." Now let´s imagine that 99.9 percent of SMD is caused by a malfunction in X that is caused by an environmental stress (this is just for the sake of argument). DISC1 malfunction, by disrupting the "X route" upstream, would end up in mental disease, but studying the role of DISC1 in neurogenesis, or its interaction with NUDEL (again for the sake of argument) would not be relevant at all. So I think that an important research avenue should be to explore all the routes in which DISC1 is involved.

The number of publications on DISC1 has been rising steadily and rapidly, from below 15 in 2004 to over 60 in 2008. This pace of discovery will certainly accelerate in the coming years. Light has been shed on many aspects of DISC1 molecular properties, expression, and functions. Mutant mice have been generated with different types of alterations in DISC1 function, and these mice have been analyzed for anatomical alterations, synaptic properties, cognitive functions, and other behaviors. New genetic studies further associated DISC1 with psychiatric disorders.

An important role in brain development for DISC1 acting at the centrosome has been established. Several of its protein interactors, also supported by recent studies, suggest other potential functions for DISC1, in other neuronal compartments. Of these, synaptic functions of DISC1 are particularly interesting, as synaptic dysfunction is thought to play an important role in psychiatric disorders. One good reason why it is worth examining plasticity related functions of DISC1 is because plasticity is ongoing throughout life, and can be therapeutically modulated.

DISC1 knockdown in adult-born dentate granule cells in vivo accelerates dendritic development. Specifically, DISC1 knockdown has been shown to accelerate the formation of dendritic spines in newborn dentate granule cells in adulthood (Duan et al., 2007). This suggests that in dentate gyrus granule cells, wild-type DISC1 normally delays the formation of dendritic spines. These findings seem at odds with findings of reduced dendritic spine density in the hippocampal formation of schizophrenia patients. Furthermore, in cell culture models, DISC1 has been shown to increase, rather than decrease, neurite outgrowth (Kamiya et al., 2005; 2006). Is it possible that the role of DISC1 in dentate granule cells is thus different than its role in other cell types?

Recent evidence supports a role of DISC1 in the activation of ERK and Akt. Specifically, in primary neuronal cultures, the knockdown of DISC1 drastically reduces the phospho-activation of ERK and Akt (about an 80 percent reduction) without affecting the total expression of either molecule (Hashimoto et al., 2006). This is interesting as much evidence suggests that similar to DISC1, ERK, and Akt activation are critical mediators of cortical neuronal migration (Chen et al., 2008; Jossin and Goffinet, 2007; Segarra et al., 2006). As a future direction, it would be interesting to determine if the role of DISC1 mutants in perturbing cortical development (e.g., Kamiya et al., 2005) is mediated by a reduction in ERK and Akt activation. Furthermore, what are the possible signaling pathways that connect DISC1 to ERK and Akt? This is particularly interesting as Akt has been involved in schizophrenia risk.

Some of the morphological abnormalities of DISC1 mutant mice are present in adulthood, but not in juvenile mice (e.g., enlarged lateral ventricles are present in mutant mice at three months but not at six weeks; Hikida et al., 2007). However, most of the behavioral characterization of DISC1 mutants has focused on adulthood. Based upon the delayed emergence of schizophrenia symptoms in humans, one would expect that mutant mice should similarly show behavioral deficits that do not emerge until adolescence/young adulthood. Is there evidence from animal models supporting this?

The online DISC1 discussion organized by Akira Sawa, Nick Brandon, and Ty Cannon and hosted by SRF on January 13, 2009, was certainly lively and nicely demonstrated how much the field has moved on since the first such discussion just two years earlier. Most gratifyingly, many new researchers have entered the fray with probing questions and fresh ideas.

If two years ago there was some lingering debate as to whether or not DISC1 was a bona fide genetic risk factor in schizophrenia, that is now well and truly settled in the positive. But the more interesting questions only partially addressed and answered in the discussion are what are the genetic mechanisms (haploinsufficiency, dominant negative), the genetic classes of variation (regulatory, copy number, missense) and their relative abundances, and what are their respective phenotypic effects at the molecular, cellular, neurological, and clinical levels? This to me is one of the main opportunities for the DISC1 field and the challenges to the rest of the field of biological psychiatry. Evidence is growing for both direct and indirect (epistatic) effects of DISC1 variation, covering the full spectrum from polymorphic variant to clear-cut mutation. But it is also clear that we should think more in terms of a DISC1 complex of proteins and therefore also the consequences of genetic variation in the constituent components of the complex to get the full picture. In order to tease out genotype-phenotype correlations, clinical studies, not just of schizophrenia, but also the affective disorders, autisms, and perhaps dementias, too, and not just clinical end phenotypes, but more importantly intermediate phenotypes, need to be integrated with genetically engineered model organism studies (mouse, zebrafish, fruit fly, etc.). This is starting to happen, but more is needed.

Whether DISC1, or rather the DISC1 pathway, is “druggable” came in late to the discussion and met with mixed opinions. My own is that the question is not whether, but how. In the search for novel antipsychotics, antidepressants, mood stabilizers, or cognitive enhancers, there seems little point ploughing the same old furrows in search of a new D2 antagonist or conventional “druggable” targets (receptors, kinases, and the like) whilst proven genetic targets are ignored. The model needs to be reworked if we are, as we surely must be, striving to transform etiological insights from DISC1 into better diagnosis and management. Encouragingly, this discussion provided hints from several directions that these questions are very much live in the minds of participants.

As for next DISC1 online discussion, the question is not whether, but when. With the pace of current progress, we won’t want to wait another two years. The year 2010 marks the 20th anniversary of the Lancet study by David St. Clair and colleagues which first described the Scottish family with a high loading of schizophrenia and related major mental illness co-segregating with a balanced t(1;11) translocation. Coincidently, it is also the 10th anniversary of the Human Molecular Genetics study in which Kirsty Millar and colleagues described the cloning of that translocation breakpoint and the discovery of DISC1. So take this as advance notice of a DISC1-focused meeting, planned for Edinburgh 2010.

In reading the online DISC1 discussion, which I unfortunately missed, I saw there were many good points concerning DISC1 biology, but there are others that need to be emphasized. The discussion rightly pointed out that DISC1 is a complex gene with potentially many post-transcriptional and post-translational modifications. The group seemed to agree that the tools used to interrogate DISC1 function therefore need to be well calibrated and validated to insure reliability and replicability of any findings. This will be important for identifying which of the many DISC1 biological pathways are relevant to its role in disease risk. This in and of itself is not an easy task, but suggestions are offered below.

The recent past in human genetics and animal models have taught us that translating common genetic variation (e.g., SNPs) into valid animal models is highly difficult. Even if unequivocal risk SNPs are identified for a given locus (to date, this seems not to be the case), the functional effects of those SNPs may be difficult to determine. It is therefore not possible to create an accurate animal model from those genetic data. Rare genetic variants, such as missense or structural mutations, are often more penetrant, the functional effects more transparent, and the creation of genetic animal models more feasible. Thus, any changes observed in animal models of these specific risk alleles are likely to be relevant to the pathogenesis of the human condition.

In the case of DISC1, there is now a long list of potential biological functions from cellular proliferation, neuronal migration, neuritic outgrowth, and intracellular signaling. Unfortunately, there is no easy method for verifying which, if any, of these functions are the critical link between the DISC1 mutation and disease risk in the Scottish family. Studies that manipulate DISC1 function (e.g., by shRNA, overexpression or otherwise) and find a certain neuronal effect often claim that this is evidence of a link between that effect and schizophrenia or depression (e.g., Abbott, 2007; Marx, 2007). It is likely, however, that these methods will identify effects that might be irrelevant to the disease process no matter how appealing the finding may be, based on current, albeit incomplete, hypotheses of pathophysiology. Unfortunately, given the efficacy of current antipsychotic and mood-stabilizing drugs and their failure to aid in the discovery of new therapeutics, responses to current medications are not likely to provide much novel insight and alone cannot be used as tools of validation.

It is thus necessary to gather much better clinical data, such as deep sequencing of DISC1, that are likely to uncover other rare mutations associated with psychiatric disorders, although this itself will not be a trivial matter, as many thousands of patients could be required to show an unequivocal statistical association. It will also be important to carefully characterize carriers of any mutations because these genetic variants may associate with symptom domains across diagnostic categories. These rare mutations can then guide the development of etiologically valid animal models of specific DISC1 risk alleles (i.e., manipulation of the endogenous mouse allele to mimic the human risk variant). Comparison of these various models along with models of other well-defined risk alleles, of which currently there are very few (see, e.g., Stark et al., 2008; Mukai et al., 2008), will likely identify common pathogenetic mechanisms. It is in this context that much of the basic work in DISC1 biology discussed by the group will be of great service.